Hand-off is a process by which a network of base stations and a mobile station maintain their communication when the mobile station travels from one coverage area to another. Generally, the coverage areas in a communication system are pre-defined by the geographical locations of its base stations, or may be defined in combination with the call loading level of base stations. Each coverage area is defined as a cell which is normally served by a base station. Furthermore, each cell is divided into several sectors such that each sector provides wireless communication in a coverage area. Very often, a hand-off is between two sectors of the same cell. In a code division multiple access (CDMA) communication system, two types of hand-off procedures are supported, namely: hard hand-off, and soft hand-off. The hand-off between two sectors of the same cell is performed by a soft hand-off routine. The communication link during a soft hand-off is transferred to a carrier which has the same frequency as the carrier frequency currently in progress. As a result, a mobile station which has commenced a soft hand-off would not disrupt its present communication link during the hand-off process. In hard handoff, the communication link which is currently in progress is disrupted for hand-off to a different carrier frequency.
In CDMA systems, a pilot channel on a specific code channel is transmitted from each base station. If a cell is divided into sectors, a pilot channel is also assigned to each sector. Typically, a cell has three to six sectors. Among other uses, the pilot channels are used to direct the mobile stations hand-off routines. Once a mobile station is moved into a coverage area, a list of all cells and sectors that are in the communication system is transmitted to the mobile station. The list contains a list of pilot signals of many cells and sectors. The mobile station constantly is searching for pilot signals of the cells and sectors that are in the list. The mobile station measures the strength of pilot signals which then are used as a criteria to create a list of possible candidate pilot signals for future hand-off. Since the list of all pilot signals is typically very long, the mobile station spends a long time scanning for all pilot signals, and in the process, consumes battery power. A long list also reduces the scan rate which results in hand-off delay. Moreover, scanning for a long list of pilot signals usually requires a complex search engine; and the search engine complexity further increases the mobile station battery power consumption. In the prior art, the mobile station limits the number of pilot signals that it scans to conserve its battery power. As a result, a limited candidate list of pilot signals is created.
The mobile station handoff process is initiated by transmitting a message to the base station which contains the list of candidate pilot signals. In CDMA system protocol, this message is called: Pilot Strength Measurement Message. The candidate pilot signals are ordered according to a priority which is generally based on the likelihood of success of completing the hand-off routine. Once the candidate list is transmitted to the base station, the base station subsequently responds back in a message, Hand-off Direction Message, to the mobile station with a direction for initiating the hand-off. Upon completion of the hand-off, the mobile station transmits a message, Hand-off Completion Message, indicating a successful completion of the hand-off process.
If a selected pilot candidate fails to complete the hand-off process during a hand-off routine, the communication system would resort to other pilot candidates in the pilot candidate list which were received in Pilot Strength Measurement Message. Since a limited candidate list is acquired, the list of pilot signals may be exhausted before the hand-off routine is successfully completed. In the case of soft hand-off process, the hand-off routine should be completed within a period of time such that the call is not interrupted and undue delay is prevented. If the list is exhausted before the hand-off process is completed, the mobile station performs either an unplanned termination of the call or tries to create a new list of candidates by searching for other pilot signals which further delays the hand-off process. In either case, the result is highly undesirable.
Therefore, there is a need for an improved method and apparatus of creating a list of pilot signal candidates for a soft hand-off routine.